Nominal statistics take into account the current exchange rate, while PPP ignores exchange rates and instead normalizes it in terms of things like Big Macs (albeit expressed in dollar terms instead). PPP is essentially an adjustment for cost of living, and so it generally makes more sense to use PPP when you're referring to per capita statistics (since you're generally trying for a sense of cost living anyways). Conversely, for comparing the size of economies or trade, you want to use nominal, as trade is conducted in nominal terms. There are reasons to go the other way, but these choices should be your first inclinations.
Purchase price parity is of course a valid view but not what I would consider in this context.